Hemostasis, the arrest of bleeding from an injured blood vessel, requires the combined activity of vascular, platelet, and plasma factors. Regulatory mechanisms counterbalance the tendency of clots to form. Hemostatic abnormalities can lead to excessive bleeding or thrombosis.

Vascular Factors

Vascular factors reduce blood loss due to trauma through local vasoconstriction (an immediate reaction to injury) and compression of injured vessels by extravasation of blood into surrounding tissues. Vessel wall injury triggers the attachment and activation of platelets and production of fibrin; platelets and fibrin combine to form a clot.

Platelet Factors

Various mechanisms, including endothelial cell nitric oxide and prostacyclin, promote blood fluidity by preventing platelet stasis and dilating intact blood vessels. These mediators are no longer produced when the vascular endothelium is disrupted. Under these conditions, platelets adhere to the damaged intima and form aggregates. Initial platelet adhesion is to von Willebrand factor (VWF), previously secreted by endothelial cells into the subendothelium. VWF binds to receptors on the platelet surface membrane (glycoprotein Ib/IX). Platelets anchored to the vessel wall undergo activation. During activation, platelets release mediators from storage granules, including
adenosinediphosphate (ADP).

Other biochemical changes resulting from activation include hydrolysis of membrane phospholipids, inhibition of adenylate cyclase, mobilization of intracellular calcium, and phosphorylation of intracellular proteins. Arachidonic acid is converted to thromboxane A2; this reaction requires cyclooxygenase and is inhibited irreversibly by aspirin and reversibly by many NSAIDs. ADP, thromboxane A2, and other mediators induce activation and aggregation of additional platelets on the injured endothelium. Another receptor is assembled on the platelet surface membrane from glycoproteins IIb and IIIa. Fibrinogen binds to the glycoprotein IIb/IIIa complexes of adjacent platelets, connecting them into aggregates.

Platelets provide surfaces for the assembly and activation of coagulation complexes and the generation of thrombin. Thrombin converts fibrinogen to fibrin. Fibrin strands bind aggregated platelets to help secure the platelet-fibrin hemostatic plug.

Plasma Factors

Plasma coagulation factors interact to produce thrombin, which converts fibrinogen to fibrin. By radiating from and anchoring the hemostatic plug, fibrin strengthens the clot.

Binds to tissue factor and is then activated to form the enzymatic component of the factor VIIa/tissue factor complex, which activates factors IX and X

Is vitamin K–dependent

VIII

Antihemophilic globulin

Is activated to factor VIIIa, a cofactor for the enzyme factor IXa in the factor IXa/VIIIa/phospholipid complex, which activates factor X

Is a large cofactor protein (as is factor V)

Circulates in plasma bound to von Willebrand factor multimers

As factor VIIIa, is inactivated by activated protein C in complex with free protein S (as is factor Va)

IX

Christmas factor

Is activated to factor IXa, the enzyme of the factor IXa/VIIIa/phospholipid complex, which activates factor X

Is vitamin K–dependent

X

Stuart-Prower factor

Is activated to factor Xa, the enzyme of the factor Xa/Va/phospholipid complex, which cleaves prothrombin to thrombin

Is vitamin K–dependent

XI

Plasma thromboplastin antecedent

Is activated to factor XIa, which activates factor IX in a reaction requiring calcium ions

Prekallikrein

Fletcher factor

Participates in a reciprocal reaction in which it is activated to kallikrein by factor XIIa

As kallikrein, catalyzes further activation of factor XII to factor XIIa

Circulates as a biomolecular complex with high molecular weight kininogen

High molecular weight kininogen

Fitzgerald factor

Circulates as a bimolecular complex with prekallikrein

XII

Hageman factor

When activated to factor XIIa by surface contact, kallikrein, or other factors, activates prekallikrein and factor XI, triggering the intrinsic coagulation pathway in vitro

XIII

Fibrin stabilizing factor

When activated by thrombin, catalyzes formation of peptide bonds between adjacent fibrin monomers, thus strengthening and stabilizing the fibrin clot

Protein C

—

Is activated by thrombin bound to thrombomodulin; then proteolyzes and inhibits (in the presence of free protein S and phospholipid) the cofactor activity of factors VIIIa and Va

Is vitamin K–dependent

Protein S

—

Circulates in plasma as free protein S and as protein S bound to C4b-binding protein of the complement system

Functions in its free form as a cofactor for activated protein C

Is vitamin K–dependent

Cell surface factors

Tissue factor

Tissue thromboplastin

Is a lipoprotein that is constitutively present on the membrane of certain tissue cells, including perivascular fibroblasts, boundary epithelial cells (eg, epithelial cells of the skin, amnion, and GI and GU tracts), and glial cells of the nervous system

May also develop in pathologic states on activated monocytes and macrophages and on activated vascular endothelium

Is present on some tumor cells

Binds factor VIIa, which initiates the extrinsic coagulation pathway

Procoagulant phospholipid

—

Acidic phospholipid (primarily phosphatidyl serine) present on the surface of activated platelets and other tissue cells

Is a component of the factor IXa/VIIIa/phospholipid complex which activates factor X and of the factor Xa/Va/phospholipid complex which activates prothrombin

Functions as the lipid moiety of tissue factor

Thrombomodulin

—

Is an endothelial cell surface binding site for thrombin, which, when bound to thrombomodulin, activates protein C

Activation of the intrinsic or extrinsic pathway activates the common pathway, resulting in formation of the fibrin clot. Three steps are involved in common pathway activation:

A prothrombin activator is produced on the surface of activated platelets and tissue cells. The activator is a complex of an enzyme, factor Xa, and 2 cofactors, factor Va and procoagulant phospholipid.

The prothrombin activator cleaves prothrombin into thrombin and another fragment.

Thrombin induces the generation of fibrin polymers from fibrinogen. Thrombin also activates factor XIII, an enzyme that catalyzes formation of stronger bonds between adjacent fibrin monomers, as well as activating factor VIII and factor XI.

Calcium ions are needed in most thrombin-generating reactions (calcium-chelating agents [eg, citrate, ethylenediaminetetraacetic acid] are used in vitro as anticoagulants). Vitamin K–dependent clotting factors (factors II, VII, IX, and X) cannot bind normally to phospholipid surfaces through calcium bridges and function in blood coagulation when the factors are synthesized in the absence of vitamin K.

Although the coagulation pathways are helpful in understanding mechanisms and laboratory evaluation of coagulation disorders, in vivo coagulation is predominantly via the extrinsic pathway. People with hereditary deficiencies of factor XII, high molecular weight kininogen, or prekallikrein have no bleeding abnormality. People with hereditary factor XI deficiency have a mild to moderate bleeding disorder. In vivo, factor XI (an intrinsic pathway factor) is activated when a small amount of thrombin is generated. Factor IX can be activated both by factor XIa and factor VIIa/tissue factor complexes.

Anticoagulants and their sites of action.

Fibrinolysis

Fibrin deposition and lysis must be balanced to maintain temporarily, and subsequently remove, the hemostatic seal during repair of an injured vessel wall. The fibrinolytic system dissolves fibrin by means of plasmin, a proteolytic enzyme. Fibrinolysis is activated by plasminogen activators released from vascular endothelial cells. Plasminogen activators and plasminogen (from plasma) bind to fibrin, and plasminogen activators cleave plasminogen into plasmin (see Figure: Fibrinolytic pathway.). Plasmin then proteolyzes fibrin into soluble fibrin degradation products that are swept away in the circulation.

Tissue plasminogen activator (tPA), from endothelial cells, is a poor activator when free in solution but an efficient activator when bound to fibrin in proximity to plasminogen.

Urokinase exists in single-chain and double-chain forms with different functional properties. Single-chain urokinase cannot activate free plasminogen but, like tPA, can readily activate plasminogen bound to fibrin. A trace concentration of plasmin cleaves single-chain to double-chain urokinase, which activates plasminogen in solution as well as plasminogen bound to fibrin. Epithelial cells that line excretory passages (eg, renal tubules, mammary ducts) secrete urokinase, which is the physiologic activator of fibrinolysis in these channels.

Streptokinase, a bacterial product not normally found in the body, is another potent plasminogen activator.

Streptokinase, urokinase, and recombinant tPA (alteplase) have all been used therapeutically to induce fibrinolysis in patients with acute thrombotic disorders.

Regulation of fibrinolysis

Fibrinolysis is regulated by plasminogen activator inhibitors (PAIs) and plasmin inhibitors that slow fibrinolysis. PAI-1, the most important PAI, inactivates tPA and urokinase and is released from vascular endothelial cells and activated platelets. The primary plasmin inhibitor is alpha2-antiplasmin, which quickly inactivates any free plasmin escaping from clots. Some alpha2-antiplasmin is also cross-linked to fibrin polymers by the action of factor XIIIa during clotting. This cross-linking may prevent excessive plasmin activity within clots.

tPA and urokinase are rapidly cleared by the liver, which is another mechanism of preventing excessive fibrinolysis.

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